Lipid aggregates such as liposomes or cationic polymers can facilitate introduction of macromolecules, such as DNA, RNA, and proteins, into living cells. Aggregates comprising cationic lipid components can be used to effect delivery of large anionic molecules, such as nucleic acids, into certain types of cells.
The use of cationic lipids has become increasingly popular since their introduction over 25 years ago. Several cationic lipids have been described in the literature and some of these are commercially available. DOTMA (N-[1-(2,3-dioleyloxy)propyl]-N,N,N-trimethylammonium chloride; IUPAC: 1,2-di-O-octadecenyl-3-trimethyl-ammonium propane (chloride salt); CAS Number: 104872-42-6) was the first cationic lipid to be synthesized for the purpose of nucleic acid transfection. DOTMA can be formulated alone or can be combined with DOPE (dioleoylphosphatidylethanolamine) into a liposome, and such liposomes can be used to deliver plasmids into some cells. Other classes of lipids subsequently have been synthesized by various groups. For example, DOGS (5-carboxyspermylglycinedioctadecylamide) was the first polycationic lipid to be prepared and other polycationic lipids have since been prepared. The lipid DOSPA (2,3-dioleyloxy-N-[2(spermine-carboxamido)ethyl]-N,N-dimethyl-1-propanaminium) has been described as an effective delivery agent.
In other examples, cholesterol-based cationic lipids, such as DC-Chol (N,N-dimethyl-N-ethylcarboxamidocholesterol) have been prepared and used for transfection. Also, 1,4-bis(3-N-oleylamino-propyl)piperazine was prepared and combined with histone H1 to generate a delivery reagent that was reported to be less toxic than other reagents. Some examples of commercially available lipids include Lipofectin® (DOTMA:DOPE) (Thermo-Fisher, Carlsbad, Calif.), LipofectAmine™ (DOSPA:DOPE) (Thermo-Fisher), LipofectAmine2000™ (Thermo-Fisher), Lipofectamine 3000 (Therom-Fisher), Fugene®, Transfectam® (DOGS), ViaFect (Promega), DNA-In, GeneIn (MTI-GlobalStem) Effectene®, and DC-Chol. Further examples are PEI polymers and dendrimers such as jetPEI (PolyPlus), and Superfect (Qiagen). None of these reagents can be used universally for all cells. This is perhaps not surprising in light of the variation in composition of the membranes of different types of cells as well as the barriers that can restrict entry of extracellular material into cells. Moreover, the mechanism by which cationic lipids deliver nucleic acids into cells is not clearly understood. The reagents are less efficient than viral delivery methods and are toxic to cells, although the degree of toxicity varies from reagent to reagent.
However, transfection agents, including cationic lipids, anionic lipids, cationic polymers, exosomes, and virasomes, are not universally effective in all cell types. Effectiveness of transfection of different cells depends on the particular transfection agent composition. In general, polycationic lipids are more efficient than monocationic lipids in transfecting eukaryotic cells. In many cases, cationic lipids alone are not effective or are only partially effective for transfection so helper lipids or transfection enhancers can be used in combination with cationic lipids.
Many biological materials are taken up by cells via receptor-mediated endocytosis, in which a surface ligand binds to a cell-surface receptor, leading to clustering of ligand-bound receptors, and formation of coated pits followed by internalization of the ligands into endosomes. Both enveloped viruses, like influenza virus and alphaviruses, and non-enveloped viruses, like Adenovirus, infect cells via endocytotic mechanisms. Enhancement of dendrimer-mediated transfection of some cells by chloroquine (a lysosomotropic agent) suggests that endocytosis is involved in at least some transfections.
Introduction of foreign DNA sequences into eukaryotic cells mediated by viral infection is generally orders of magnitude more efficient than transfection with anionic lipids, cationic lipid, PEI, peptides, or dendrimer transfection agents. Viral infection of all the cells in a culture requires fewer than 10 virus particles per cell. Although the detailed mechanism of fusion is not fully understood and varies among viruses, viral fusion typically involves specific fusogenic agents, such as viral proteins, viral spike glycoproteins and peptides of viral spike glycoproteins. Cell binding and internalization also can be enhanced, accelerated or made selective with peptides that bind cell receptors. For example, the penton-base protein of the Adenovirus coat contains the peptide motif RGD (Arg-Gly-Asp) which mediates virus binding to integrins and viral internalization via receptor-mediated endocytosis.
The efficiency of cationic lipid transfections has been shown to be enhanced by the addition of whole virus particles to the transfection mixture. Certain viral components may also enhance the efficiency of cationic lipid-mediated transfection. For example, it has been suggested that “Lipofectin™”-mediated transfections may be enhanced 3-4-fold by adding influenza virus hemagglutinin peptides to the transfection mixture. Antibodies have been shown to enhance cationic lipid transfections and transferrin-poly lysine or asialoglycoprotein polylysine have been shown to enhance cationic lipid transfection.
Nevertheless, these methods do not work for all cell types, require relatively complex protocols and are inconvenient. It is apparent, therefore, that new and improved methods for introducing macromolecules, and particularly nucleic acids, into cell, are greatly to be desired. In particular, improved methods for introducing nucleic acids into a wider variety of cells, and particularly into primary cells, are greatly to be desired.